At the end of the 19th century, Austrian physician and neurologist (神经病学家) Sigmund Freud developed a new theory that explained psychology. Freud argued that the mind had deep emotional desires hidden from consciousness. He termed the mental storehouse of hidden desire the unconscious. He claimed that the unconscious was a place of irrational (不合理的), often sexual and aggressive, desires. It was, he believed, also the source of human volition (意志；意志力) and will.The unconscious was a revolutionary idea, inl9th-century psychology, which commonly asserted that the individual was an entirely rational, self-conscious being. Freud suggested that because the unconscious was the source of human will, truly rational individual behavior was impossible unless the individual came to understand the unconscious wellsprings of behavior. Freud developed psychoanalysis (精神分析学), a method of exploring the unconscious that relied on the analysis of dreams and other irrational phenomena, such as slips of the tongue. But because the unconscious was entirely hidden from individual consciousness, the key to understanding it was the participation of another person or a trained analyst in the process of unconscious discovery. Freud’s theory was and remains controversial. It however had a tremendous influence on art, education, history, psychology, religion, and sociology. His subsequent work on human sexuality and the structure of the mind, along with the application of his work to social and political problems, made him one of the most important intellectual figures of the 20th century. Freud suggested that because the unconscious was the source of human will, truly rational individual behavior was impossible（）.

A. unless the individual came to understand the unconscious sources of behavior
B. unless the individual came to understand the unconscious understanding of behavior
C. unless the individual came to understand the unconscious reason of behavior
D. unless the individual came to understand the unconscious effect of behavior